Back when I started this mod I had the same problem, RPM variations, I was thinking to get everything together and put it on the trash can
.
But I whent back to the diagrams , for some reason powering the IC chip directly from the PSU was the cause of my variation.
It was when I Powered the IC chip from the motherboard the problem whent away, using both negative and positive leads from the motherboard.
You also may want to make sure you have the proper conection at the pump.